1. Field of the Invention
The present invention relates to a fibre-type light wavelength conversion device.
2. Description of Background Information
An optical pickup is known by which high density writing and reading of information on and from a disc are enabled by changing the wavelength of a laser beam emitted from a laser source to a half the original wavelength by means of a light wavelength conversion device (reference is directed to Japanese Patent Application Laid-Open No. 61-50122).
As the light wavelength conversion device for use in this type of pickup, there is an optical fibre-type SHG (Second Harmonics Generator) using a second-order non-linear optical effect. A phase matching of Cerenkov radiation system is adopted in the optical fibre type SHG. With this system, it is possible to generate a second harmonic wave (referred to as SH wave hereinafter) whose phase matching is attained almost automatically. The general concept of this device is shown in FIGS. 1A and 1B.
In FIG. 1A, when the fundamental wave mode is propagated through the core with the effective reflective index of N (.omega.), the non-linear polarizing wave generating the SH wave is also propagated at the same phase velocity C/N (.omega.) (C is the speed of light). It is assumed that this non-linear polarizing wave produces the SH wave in a direction making an angle .theta. with respect to the direction of the wave guide at a point A, and generates the SH wave in the direction of .theta. as before at a point B, after the elapse of a unit time. If the SH wave generated at the pont A propagates through the clad and reaches to a point C after the elapse of the unit time and the angle .theta. is such an angle that lines AC and BC are perpendicular to each other, then the plane of the SH wave which is generated from the non-linear polarized wave becomes equal to BC, and as a reuslt, a coherent SH wave is generated.
The condition of the phase matching is, according to the figure, as follows: EQU N(.omega.)=N.sub.clad (2.omega.) cos .theta. (1)
where N.sub.clad (2.omega.) is the refractive index of the clad for the SH wave. This in turn gives, EQU N(.omega.)&lt;N.sub.clad (2.omega.) (2)
The above equation means that the SH is generated automatically in the direction where the phase matching is performed when at least the condition mentioned by the equation (2) is satisfied. Generally, with the refractive indices of the clad and core for the fundamental wave being n.sub.clad (.omega.) and n(.omega.) and with air as the over-layer, the condition for the fundamental wave to propagate through the core as a mode is expressed as follows: EQU N.sub.clad (.omega.)&lt;N(.omega.)&lt;n(.omega.) (3)
Wavelength dispersion of the clad's refractive index will now be considered, Since n.sub.clad (.omega.)&lt;n.sub.clad (.omega.), the equation (2) is satisfied for all of the fundamental wave modes irrespectively of the diameter of the core so far as the following expression (4) is satisfied. EQU N.sub.clad (.omega.)&lt;N(.omega.)&lt;n.sub.clad (2.omega.) (4)
Moreover, there are fundamental modes satisfying the equation (2) in a certain range of the diameter of the core even under the following condition. EQU N.sub.clad (.omega.)&lt;n.sub.clad (2.omega.)&lt;n(.omega.)
The second harmonic wave generated in this way is propagated in a clad mode as illustrated in FIG. 1B in which total reflection occurs repeatedly at the boundary between the clad and air. Then, the second harmonic wave is emitted in conical shape from the end of fibre in directions making an angle .theta. relative to the fibre's direction. The equiphase front of the second harmonic wave emitted in this way is in a conical surface with an axis on the central axis of the fibre.
In this fibre-type light wavelength conversion device, the phase of the SH wave will be inverted if the reflection of the SH wave occurs between the boundary surface of the cladding layer and air while it propagates through the cladding layer. Hence, for an emerging SH wave ring illustrated in FIG. 1B, the inner SH wave will have a phase difference of 180.degree. relative to the outer SH wave by the above-described reflection at the boundary surface.